Pyrotechnic Grains of Large Dimensions, and Their Production and Use

ABSTRACT

The present invention provides pyrotechnic grains of composition comprising at least one oxidizing charge and at least one reducing charge and no binder. In characteristic manner, said grains are in the form of substantially cylindrical blocks:
         with a thickness of more than 5 mm;   with an equivalent diameter of 10 mm or more; and   with porosity in the range 1% to 8%, limits included.       

     Said grains, advantageously based on guanidine nitrate and basic copper nitrate, are suitable for the slow, low-pressure generation of gas over an extended period.

The present invention relates to large pyrotechnic grains based onbinder-free formulations, to their production, and to their use. Moreprecisely, it relates to:

-   -   pyrotechnic grains which are in the form of blocks (monolithic        blocks) with large dimensions;    -   a method of producing them; and    -   a method of generating gas comprising the combustion of said        grains.

Gas generation has a number of applications, both in the civilian and inmilitary fields.

In the automotive safety sector, gas generators have been developed witha short operational duration, which use compositions with a highcombustion rate. Those compositions are in the form of thin pellets (1mm [millimeter] to 3 mm) with a small diameter (3 mm to 7 mm). Thisprovides them with a large combustion surface, encouraging a high rateover a short period (necessary for airbag applications in automotivesafety, of the driver's airbag type). The porosity of such pellets isgenerally less than 4%. Pelletization operations carried out to producesaid pellets are well known, similar to those used in medicinalformulations. Compositions based on guanidine nitrate and basic coppernitrate have been used in that context (United States patent U.S. Pat.No. 5,608,183). An attempt has even been made to improve the combustionrate (U.S. Pat. No. 6,143,102) since said combustion rate of suchcompositions is rather low, compared with that of other compositionsused in the same field.

In similar sectors (automotive safety, to inflate curtain types ofairbag which need to remain inflated for a long period), or incompletely different sectors, gas generators have been proposed,especially for:

-   -   inflating or deforming a structure;    -   propelling a liquid or a gas;    -   actuating a jack or some other mechanical actuator;    -   causing a displacement.

Generating and supplying gas has also been exploited to operate fireextinguishing devices (FR-A-2 864 905 and FR-A-2 870 459).

That relates to the field of slow gas generation. For slowly generatingand supplying gas under pressure, a gas stored under pressure isgenerally used. The use of chemical reactions or the combustion ofpyrotechnic cartridges has also been mentioned. With the combustion ofpyrotechnic cartridges, the flow rate of the gas that is generated iscontrolled by selecting the characteristics of the propellant, thegeometry of the cartridges, and/or the conditions for ejecting saidgenerated gas, etc.

The present invention falls within the context of generators operationproviding long duration, low flow rate, and low operating pressure.

In such a context, the present invention proposes pyrotechnic grainsthat are simple in shape and that satisfy the following specifications:

a) a long combustion period (50 ms [millisecond] to 1 min [minute]) dueto a slow combustion rate (to ensure a long period of operation) and/orto a ratio of combustion area over burning thickness that is low, and/orto a low operating pressure (for use in low pressure generators and toencourage a low combustion rate);

b) low dependency of the rate of combustion on the operating temperatureof the grain, to ensure good ballistic behavior under extremetemperature conditions from −60° C. to 110° C. (hence an extension ofthe application range (see below), especially in aeronautics);

c) suitable mechanical behavior (without the intervention of a binder,the presence of which would deleteriously affect ballistic performance),especially encouraging the use of said grains in an aggressive vibratoryenvironment.

The pyrotechnic grains of the invention are also advantageouslynon-aggressive for the gas generator in which they are used and for theassociated system (i.e., on combustion, they generate neither particlesnor corrosive species). In the same manner, the generated species areadvantageously of low toxicity.

Referring to point a) of the above specifications, the skilled personwill understand that the pyrotechnic grain of the invention must bethick and have a small combustion area. Thus, it must be a large grain,larger than the pellets mentioned above.

Referring to point b) of the above specifications, the skilled personwill know that said grain must have low porosity.

Said skilled person will also be aware of the difficulties infabricating a large grain of that type (pyrotechnic grain) with lowporosity, with appropriate mechanical behavior, and without using abinder. He knows that pyrotechnic grains of large dimensions and withoutbinder in their composition have generally porosity values which are notacceptable in reference to their ballistic properties, more particularlytheir combustion rate. This point is all the more crucial than theiroperating temperature is high.

Surprisingly, the Applicant has obtained pyrotechnic grains, fromspecific oxidizing and reducing charges, which are simple in shape andthat satisfy the above specifications. Such pyrotechnic grainsconstitute the first aspect of the present invention.

Said pyrotechnic grains of the invention have a composition thatcomprises at least one (specific) oxidizing charge and at least one(specific) reducing charge, but that includes no binder. In thisrespect, they are pyrotechnic grains of the pellet type mentioned above.The composition of the pyrotechnic grains of the invention generallyessentially consists of at least one such (specific) oxidizing chargeand at least one such (specific) reducing charge. It generallyessentially consists of a (specific) oxidizing charge (an oxidizer) anda (specific) reducing charge (a reducer).

Said at least one specific oxidizing charge is selected from ammonium,potassium, sodium, barium, strontium, and basic copper nitrates, andammonium, potassium, and sodium perchlorates, and mixtures thereof.Advantageously, it comprises basic copper nitrate and/or potassiumperchlorate. Highly advantageously, it consists of basic copper nitrateor potassium perchlorate. Preferably, it consists of basic coppernitrate.

Said at least one specific reducing charge is selected fromnitroguanidine, guanidine nitrate, and mixtures thereof. Preferably, itconsists of guanidine nitrate.

In characteristic manner, said pyrotechnic grains of the invention arein the form of substantially cylindrical blocks:

-   -   with thickness of more than 5 mm;    -   with an equivalent diameter of 10 mm or more; and    -   with porosity in the range 1% to 8%, limits included.

Said (monolithic) blocks are large blocks, which are simple in shape andof low porosity.

Said blocks have a substantially cylindrical shape. Generally, but notexclusively, they are circular cylinders or near (quasi) circularcylinders.

The shape of the grains of the invention is the signature of the laststep of the method by which they are obtained: a compacting step (seebelow).

Said blocks may in particular consist of solid (quasi) cylinders or(quasi) cylindrical sleeves.

The grains in question are large objects (thickness more than 5 mm,equivalent diameter (or, clearly, diameter if it is a perfect circularcylinder) of 10 mm or more), and are relatively dense (porosity in therange 1% to 8%).

Non-limiting dimensions of said grains are detailed below.

Generally and independently:

-   -   the thickness of the blocks is 75 mm or less; or    -   the equivalent diameter of said blocks is 75 mm or less.

In general, said thickness and said equivalent diameter are both 75 mmor less.

According to a first advantageous embodiment, independently

-   -   the thickness of the blocks is 7.5 mm or more; or    -   the equivalent diameter of said blocks is 20 mm or more.

In general, in the context of said first advantageous embodiment, saidthickness and said equivalent diameter are both as precised above.

According to a second advantageous embodiment, independently:

-   -   the thickness of the blocks is in the range 10 mm to 60 mm        (values of 10 mm and 60 mm being included); or    -   the equivalent diameter of said blocks is in the range 10 mm to        60 mm (values of 10 mm and 60 mm being included).

In general, in the context of this second advantageous embodiment, saidthickness and said equivalent diameter are both in the range 10 mm to 60mm (values of 10 mm and 60 mm being included).

In particular, pyrotechnic blocks of the solid circular cylinder typeand with the dimensions given below, have been produced in the contextof the invention:

-   -   diameter of 25 mm and thickness of 10 mm;    -   diameter of 50 mm and thickness of 50 mm.

Further, it has been stated that the porosity of the blocks is in therange 1% to 8% (it should be noted here that this parameter, expressedas a percentage, corresponds to the ratio between the actual density andthe theoretical density; it is in fact the difference from thetheoretical density).

Said porosity is advantageously 5% or less. It should be recalled thatthe lower said porosity, the lower the dependency of the rate ofcombustion on the operating temperature of the grain.

It should be indicated here that, in a non-limiting manner, blocks,which are of the circular cylinder type with the dimensions describedabove, have been respectively obtained with the porosities indicatedbelow:

-   -   porosity of about 5% (with diameter=25 mm and thickness=10 mm);    -   porosity of about 7% (with diameter=thickness=50 mm).

As indicated above, the pyrotechnic grains of the invention, which aresimple in shape, being substantially cylindrical, may in particularconsist of solid cylinders or sleeves. The equivalent internal diameterof said sleeves is advantageously more than 10 mm; preferably, it is inthe range 12 mm to 35 mm (that implies obviously a greater equivalentdiameter of the sleeve).

Referring to the composition of the pyrotechnic grains of the invention,the following non-limiting details can furthermore be given.

The said composition of the pyrotechnic grains of the inventionadvantageously includes basic copper nitrate as the oxidizing charge andguanidine nitrate as the reducing charge. Highly advantageously, itcomprises basic copper nitrate as the only oxidizing charge andguanidine nitrate as the only reducing charge. In the automotive safetyfield (driver type airbags), said basic copper nitrate/guanidine nitratecombination is known to have a combustion rate that is rather lowrelative to that of other compositions used in this field. In thecontext of this highly advantageous embodiment, said compositiongenerally comprises:

-   -   45% to 55% by weight of guanidine nitrate;    -   40% to 50% by weight of basic copper nitrate; and    -   0 to 5% by weight of additives.

The additives in question, which are optionally present, are chemicalcompounds that are known per se as well as for their beneficialinvolvement in the general production of pyrotechnic grains. Inparticular, they may be:

-   -   slagging agents supplied with the starting materials: oxidizing        charge(s) and reducing charge(s) (for example: alumina);    -   processing aids of the pressing aid or unmolding aid type (for        example: silica, calcium stearate, mica, etc).

In a second aspect, the present invention provides the production oflarge pyrotechnic grains, as described above.

The method in question comprises a series of steps that are known perse.

Surprisingly, it has been possible to find operating conditions for thisseries of steps (which are known per se) that enable, from the selectedoxidizing and reducing charges, grains of the invention to be obtainedwithout the involvement of a binder, and having simultaneously largedimensions, low porosity, and also acceptable mechanical properties(which can tolerate the vibrational environment of an automobile oraircraft).

The method of producing pyrotechnic grains of the invention comprisescarrying out the following steps in succession:

-   -   intimately dry or wet mixing said oxidizing and reducing charges        used in the powder state;    -   dry or wet granulating the powdered mixture obtained;    -   sizing the granules obtained; and    -   shaping the retained sized granules by compacting under        conditions that produce the anticipated result, i.e. the        production of pyrotechnic grains having the dimensional and        porosity characteristics defined above.

The conditions in question are principally

-   -   the characteristics of the starting powders;    -   the characteristics of the retained sized granules; and    -   the compacting parameters.

The Applicant has discovered that suitable conditions may be combined toobtain the desired result.

Before detailing said conditions (parameters) of the method of theinvention, it should in general be indicated:

-   -   that the powders are advantageously intimately dry mixed    -   that granulation is also advantageously a dry granulation        (independently of the manner of said intimate mixing);    -   that the granules are advantageously sized by sieving; and    -   that at least one shaping additive (a pressing aid, such as        calcium stearate or mica) may advantageously be added to said        sized granules before shaping them by compacting.

In characteristic manner, to obtain the anticipated result, moreparticularly to obtain a slow rate of combustion, at least one of thecharges used in the powder state has a median diameter that issubstantially larger than that which is used in the prior art to preparepellets (of small dimensions). Thus, it is recommended in the context ofcarrying out the method of the present invention to use at least part ofthe reducing charge(s) with a median diameter that is substantiallyhigher (×4, ×6) than that known to be used in the preparation of priorart pellets. It is recommended that a reducing charge with a mediandiameter of 200 μm [micrometer] or more be used. In particular, it isrecommended that guanidine nitrate with a median diameter of 200 μm to300 μm be used. Said guanidine nitrate is conventionally used for thefabrication of pellets, with a median diameter of 50 μm.

In characteristic manner, to obtain the anticipated result, moreparticularly to obtain low porosity, the granules selected are thosethat have a median diameter of more than 200 μm, preferably in the range500 μm to 1000 μm. It is thus recommended, in the context of the presentinvention, that basic copper nitrate/guanidine nitrate granules with amedian diameter in the range 500 μm to 700 μm or potassiumperchlorate/guanidine nitrate granules with a median diameter in therange 500 μm to 1000 μm be selected.

The skilled person is capable of optimizing this parameter (granulesize) as well as the others (powder mean diameter and compactingparameters) as a function of the exact nature of the products inquestion.

Regarding the compacting parameters, in general, to obtain theanticipated result, more particular to obtain low porosity, a pressurein the range 15×10⁶ Pa [pascal] to 35×10⁶ Pa is applied for 0.1 s[second] to 30 s.

Compacting is clearly carried out in a mold the shape of whichcorresponds to that envisaged for the prepared pyrotechnic grain(substantially cylindrical shape).

The method of the invention is very advantageously carried out with allthe above advantageous embodiments, i.e.

-   -   a median diameter of the at least one reducing charge of 200 μm        or more;    -   a median diameter of the granules of more than 200 μm; and    -   a compacting step carried out at 15-35×10⁶ Pa during 0.1 to 30        s.

The method of the invention is advantageously carried out with acombination of basic copper nitrate or potassium perchlorate/guanidinenitrate; highly advantageously, it is carried out with a basic coppernitrate/guanidine nitrate combination. In the context of the presentinvention, the combustion rate of compositions based on these twoelements (BCN/NG) is reduced. The teaching of the present invention thusgoes against that of U.S. Pat. No. 6,143,102.

In a third aspect, the present invention provides a method of generatinggas, the method comprising, in conventional manner, the combustion of atleast one pyrotechnic grain. In characteristic manner, said method ofthe invention comprises the combustion of at least one pyrotechnic grainas described above (large pyrotechnic grain) and/or produced using themethod described above.

Said gas generation method is carried out in a suitable device which, inparticular, has a combustion chamber adapted to the dimensions of theoriginal pyrotechnic charge, based on large pyrotechnic grains. Incommon with currently known devices, said device comprises:

-   -   elements for retaining, positioning, and adjusting the        pyrotechnic charge based on large pyrotechnic grains (which        constitutes the first aspect of the present invention);    -   ignition elements;    -   at least one safety cap ensuring depressurization of the        generator in the event of accidental overpressure;    -   at least one nozzle; and    -   a filter to retain the majority of the solid combustion        residues.

Interesting applications for the pyrotechnic grains of the invention arespecified below, i.e. fields in which gas generation must be maintainedfor a relatively long period.

The following applications are particularly contemplated

-   -   the generation of gas intended to propel a fluid, either        directly or via a separating membrane, for example, to        extinguish fire in civilian or military aircraft engines, to        inert bays in civilian or military aircraft, to extinguish fire        in a building, in all types of civilian vehicles (automobiles,        etc) or military vehicles (tanks, etc), to inject grease or        active principle, etc;    -   the generation of gas in a pyrotechnic actuator intended to        actuate a jack, for example (to open an aircraft door, for        example);    -   the generation of gas to inflate or deform inflatable structures        that are deformable, such as aircraft escape chutes, automotive        safety anti-submarining structures, or releasers.

Finally, examples of the present invention are described below.

EXAMPLE 1

Pyrotechnic grains of the invention were prepared from the followingstarting materials:

-   -   basic copper nitrate (BCN): powder with a median diameter of 8        μm;    -   guanidine nitrate (GN): powder with a median diameter of 218 μm;    -   alumina (slagging agent): powder with a median diameter of 1 μm.

Said powders were dry mixed in a Sofraden® type powder mixer (a Guédu®type mixer would also have been suitable) in the following proportions:

BCN 44.87%

GN 52.44%

Alumina 2.69% (weight %).

The intimate mixture of powders obtained was dry granulated in a SahutConreur® type granulator (an Alexanderwerk® type granulator would alsohave been suitable). Said intimate mixture of powders was forced througha screen. The granulometric distribution of the granules obtained wasfrom 10 μm to 1000 μm.

The granules obtained were then sieved, and those with a mean diameterof more than 200 μm were selected.

Said selected granules were then compacted on a 35 (metric) tonne singleaction press. The compacting pressure applied was 230 bars (23×10⁶ Pa).It was applied for 10 s.

Pyrotechnic grains in accordance with the invention were obtained, whichgrains were cylindrical monoliths (shape of mold) with a thickness of 10mm, a diameter of 24.6 mm, and porosity of 3%.

EXAMPLE 2

In similar manner, cylindrical grains were prepared as follows:

-   -   with a thickness of 50 mm;    -   with a diameter of 50 mm;    -   with porosity of 7%.

EXAMPLE 3 Prior Art

In similar manner, prior art pellets, which were cylindrical grains,were prepared as follows:

-   -   with a thickness of 1.8 mm;    -   with a diameter of 6.3 mm;    -   with porosity of 2%.

The grains of said Examples 1 to 3 were tested in generators with thesame architecture. The graphs of the operating pressures of saidgenerators as a function of time are given in the accompanying FIGS. 1to 3.

With the prior art pellet, as expected, an operating peak was obtained(FIG. 3). With the blocks of the invention, the desired spread over timewas observed (FIGS. 1 and 2).

Further, the mechanical behavior of the grains of the invention asregards aeronautical vibrations was tested, with satisfactory results(standard RTCA DO-160D: “Environmental Conditions and Test Proceduresfor Airborne Equipment”).

1. A pyrotechnic grain, the composition of which comprises at least oneoxidizing charge selected from ammonium, potassium, sodium, barium,strontium, and basic copper nitrates, and ammonium, potassium, andsodium perchlorates, and mixtures thereof; at least one reducing chargeselected from nitroguanidine, guanidine nitrate, and mixtures thereof;and no binder, wherein it is in the form of a substantially cylindricalblock: with a thickness of more than 5 mm; with an equivalent diameterof 10 mm or more; and with porosity in the range 1% to 8%, limitsincluded.
 2. The pyrotechnic grain according to claim 1, wherein thethickness of said block is 7.5 mm or more and/or the equivalent diameterof said block is 20 mm or more.
 3. The pyrotechnic grain according toclaim 1, wherein the thickness and/or equivalent diameter of said blockis in the range 10 mm to 60 mm, limits included.
 4. The pyrotechnicgrain according to claim 1, wherein the porosity of said block is 5% orless.
 5. The pyrotechnic grain according to claim 1 wherein it is in theform of a sleeve of internal equivalent diameter that is advantageouslymore than 10 mm.
 6. The pyrotechnic grain according to claim 1, whereinsaid at least one oxidizing charge consists of basic copper nitrate. 7.The pyrotechnic grain according to claim 1, wherein said at least onereducing charge consists of guanidine nitrate.
 8. The pyrotechnic grainaccording to claim 1, wherein its composition comprises: 45% to 55% byweight of guanidine nitrate; 40% to 50% by weight of basic coppernitrate; and 0 to 5% by weight of additives.
 9. A method of producing apyrotechnic grain according to claim 1, the method comprising:intimately dry or wet mixing said oxidizing and reducing charges used inthe powder state; dry or wet granulating the powdered mixture obtained;sizing the granules obtained; and shaping the retained sized granules bycompacting; the characteristics of the starting powders, the retainedsized granules and the compacting parameters being selected to producesaid pyrotechnic grains having said dimensional and porositycharacteristics.
 10. The method according to claim 9, wherein theintimate mixture or (and) granulation is (are) carried out by a drymethod.
 11. The method according to claim 9, wherein the granules aresized by sieving.
 12. The method according to claim 9, wherein itcomprises: using at least one reducing charge with a median diameter of200 μm or more; and/or, advantageously and sieving the granules toretain those with a median diameter of more than 200 μm; and/or,advantageously and shaping by compacting at a pressure in the range15×10⁶ Pa to 35×10⁶ Pa for 0.1 s to 30 s.
 13. The method according toclaim 9, wherein it is carried out with guanidine nitrate as thereducing charge and basic copper nitrate or potassium perchlorate as theoxidizing charge.
 14. A method of generating gas, comprising thecombustion of at least one pyrotechnic grain, wherein said at least onepyrotechnic grain is a grain according to claim 1 and/or a grainproduced by a method comprising: intimately dry or wet mixing saidoxidizing and reducing charges used in the powder state: dry or wetgranulating the powdered mixture obtained: sizing the granules obtained:and shaping the retained sized granules by compacting: thecharacteristics of the starting powders, the retained sized granules andthe compacting parameters being selected to produce said pyrotechnicgrains having said dimensional and porosity characteristics.